Cable television (CATV) systems were originally intended to deliver analog television signals to homes in an essentially unidirectional manner. However, the coaxial cable used to carry these signals can support bidirectional transmission of signals, i.e., transmission both to and from the home. As a result, cable television companies have begun to provide data communications services to meet consumers' growing demand for high speed data transmission over computer networks, such as the Internet.
Devices known as cable modems have been developed to convert digital data to a modulated radio frequency (RF) signal, and vice versa, for transmission on a CATV system. This conversion is performed both by a cable modem at the subscriber's home and, on the other end, by headend equipment handling multiple subscribers. Thus, the subscriber receives data from a computer network using a downstream channel and transmits data to the network using an upstream channel.
As computer networks deliver increasing amounts of content to users, a need has arisen for increased throughput and communications robustness. It is also essential that communications equipment operate over a wide range of channel impairments, which can include, for example, ingress noise, burst noise, impulse noise, linear distortions, non-linear distortions, and adjacent channel interference. Moreover, the different types of services that use CATV systems give rise to additional needs. For example, for data services, such as Internet access, a high packet throughput and a low packet error rate are primary considerations. For such applications, low delay is not a key consideration. For multimedia services, such as speech, video, and telephony, however, low delay is the primary consideration. It is desirable to minimize the periods of a high bit error rate (BER) for these applications. Additionally, compatibility with existing standards and equipment is important for all applications.